Aliphatic isocyanates which have uretdione groups and are based on optionally branched, linear-aliphatic diisocyanates have a particularly low viscosity. Products based on cycloaliphatic diisocyanates generally range from highly viscous to solid substances which can be used as emission-free, internally blocked crosslinkers in coating systems.
An overview of isocyanate oligomerization is given in J. Prakt. Chem./Chem. Ztg. 1994, 336, 185-200.
Tris(dialkylamino)phosphines (DE-A 3 030 513), if appropriate in combination with cocatalysts (DE-A 3 437 635), display a good selectivity for the formation of uretdione groups (uretdione selectivity). However, the serious problem of the high cancer-causing potential of their phosphorus oxides, e.g. hexamethylphosphoramide, stands in the way of their industrial use.
DE-A 3 739 549 discloses catalytic NCO dimerization using 4-dialkylaminopyridines, e.g. 4-dimethylaminopyridine (DMAP), but uretdione formation proceeds selectively only in the case of specific cycloaliphatic isocyanates such as isophorone diisocyanate (IPDI). Linear-aliphatic isocyanates such as hexamethylene diisocyanate (HDI) and branched, linear-aliphatic isocyanates such as trimethylhexane diisocyanate (TMDI) and methylpentane diisocyanate (MPDI) give mainly strongly coloured, heterogeneous reaction products when DMAP and related compounds are used.
DE-A 1 670 720 discloses the preparation of aliphatic polyisocyanates having uretdione groups, with tertiary phosphines having at least one aliphatic substituent and also boron trifluoride and its adducts being used as catalysts. It is indicated that high proportions of uretdione groups in the product can be obtained only at low conversions and reaction temperatures in the range from 50 to 80° C., with isocyanate trimers (isocyanurates and iminooxadiazinediones) and, especially at elevated temperature, other by-products such as carbodiimides or uretonimines being simultaneously formed. Uretonimines are a particular problem since they tend to liberate monomeric isocyanate during storage.
DE-A 10254878 describes the use of phosphines having at least one cycloaliphatic, P-bonded radical as catalysts for NCO dimerization. These catalysts display a significantly higher uretdione selectivity than other trialkylphosphines of the prior art. The use of a special case of these phosphines, namely representatives having bicyclic radicals, for the same use is described in DE 10354544.